The breast cancer susceptibility gene contains at its C-terminus two copies of a conserved domain that was named BRCT for BRCA1 C-terminus. This domain of about 95 amino acids is found in a large variety of proteins involved in DNA-repair, recombination and cell cycle control [1,2,3]. The BRCT domain is not limited to the C-termini of protein sequences and can be found in multiple copies or in a single copy as in RAP1 and TdT. Recent data [4] indicate that the BRCT domain functions as a protein-protein interaction module.

The structure of the first of the two C-terminal BRCT domains of the human DNA repair protein XRCC1 has recently been determined by X-ray crystallography [4].

Some proteins known to contain a BRCT domain are listed below:

• Mammalian breast cancer type 1 susceptibility protein. It may regulate gene expression.
• Human p53-binding protein 1.
• Human poly(ADP-ribose) polymerase (PARP) (EC 2.4.2.30). It modifies various nuclear proteins by poly(ADP-rybosyl)ation.
• Vertebrate DNA nucleotidylexotransferase (EC 2.7.7.31). It adds nucleotides at the junction (N region) of rearranged Ig heavy chain and T cell receptor gene segments during the maturation of B and T cells.
• Mammalian DNA-repair protein XRCC1.
• Human DNA ligase III (EC 6.5.1.1). It is involved in DNA strand-break repair.
• Human DNA ligase IV (EC 6.5.1.1).
• Drosophila germline transcription factor 1. A putative transcription factor active during oogenesis and embryogenesis.
• Baker's yeast DNA ligase II (EC 6.5.1.1).
• Baker's yeast RAD9 protein. It is essential for cell cycle arrest following DNA damage by X-irradiation or inactivation of DNA ligase.
• Baker's yeast RAP1. It is involved in telomeric and HM loci silencing.
• Escherichia coli DNA ligase (EC 6.5.1.2). It is essential for DNA replication and repair of damaged DNA.
• Synechocystis sp. DNA ligase (EC 6.5.1.2). It is probably essential for DNA replication and repair of damaged DNA.